Regulating surface active hydrogen on CeO₂ for efficient electrochemical synthesis of hydroxylamine from nitrates

Electrocatalytic nitrate reduction (NO₃^-RR) offers a highly promising alternative route for synthesizing hydroxylamine (NH₂OH) under mild conditions. However, this process faces intense competition from the hydrogen evolution reaction (HER), resulting in suboptimal NH₂OH yields and Faradaic efficiencies. This study reports a highly efficient electrocatalytic synthesis of hydroxylamine using a hydrogen-annealed CeO₂ catalyst rich in oxygen vacancies (H₂-CeO₂). The study shows that oxygen vacancies not only optimize the surface electronic structure of the catalyst and enhance the adsorption of nitrate ions and reaction intermediates, but also modulate the adsorption behavior of active hydrogen, thereby facilitating its involvement in hydrogenation steps while suppressing the competing HER. Compared to the reversible hydrogen electrode (vs. RHE), the H₂-CeO₂ catalyst achieves a Faraday efficiency of 75 ± 2% at -1.2 V and exhibits a high productivity of 459 ± 21 μmol cm^-2 h^-1 at -1.4 V. Through a series of in situ spectroscopic studies and theoretical calculations, we elucidate the intrinsic mechanism whereby oxygen vacancies modulate the adsorption-desorption equilibrium of H*, redirecting the reaction pathway from hydrogen evolution to hydrogenation, thereby enabling efficient NH₂OH synthesis. This work offers a defect-engineering strategy to regulate active hydrogen intermediates for designing high-performance electrocatalysts.